Method of applying a coating by cathode sputtering

ABSTRACT

THE INVENTION RELATES TO A METHOD OF COATING A LAYER OF INORGANIC, SOLID MATERIAL UPON A BASE BY CATHODE SPUTTERING. THE INVENTION IS CHARACTERIZED BY THE FACT THAT A GRID OR SIEVE-LIKE PERFORATED CATHODE, WHICH IS COMPRISED, AT LEAST AT ITS SURFACE, OF THE MATERIAL IS TO BE DEPOSITED, IS ARRANGED BETWEEN ANODE AND SUBSTRATE AND IS NEGATIVELY BIASED AGAINST BOTH. THE SUBSTRATE IS BROUGH SO CLOSE TO THE CATHODE THAT THE SPACE BETWEEN THE TWO IS INSUFFICIENT TO MAINTAIN AN INDEPENDENT GAS DISCHARGE, AT THE ADJUSTED PRESSURE AND THE APPLIED VOLTAGE.

April 4, 1972 F. KRAUS 3,654,110

METHOD OF APPLYING A COATING BY CATHODE SPUTTERING Filed Feb. 6, 1969 Hm'lm United States Patent 3,654,110 METHOD OF APPLYING A COATING BYCATHODE SPUTTERING Friedrich Kraus, Neubiherg, Germany, assignor toSiemens Aktieugesellschaft, Berlin and Munich, Germany Filed Feb. 6,1969, Ser. No. 797,125 Claims priority, application Germany, Feb. 12,1968, P 16 90 692.6 Int. Cl. C23c 15/00 US. Cl. 204-192 11 ClaimsABSTRACT OF THE DISCLOSURE The invention relates to a method of coatinga layer of inorganic, solid material upon a base by cathode sputtering.The invention is characterized by the fact that a grid or sieve-likeperforated cathode, which is comprised, at least at its surface, of thematerial is to be deposited, is arranged between anode and substrate andis negatively biased against both. The substrate is brought so close tothe cathode that the space between the two is insuificient to maintainan independent gas discharge, at the adjusted pressure and the appliedvoltage.

It is known to use the metal particles sputtered during an electricdischarge, for metallizing such objects as a semiconductor surface. As arule, the object to be coated, if it is conductive, is connected as theanode and the material to be sputtered as the cathode of a gasdischarge, which discharge is effected at an appropriately high voltage.If the body to be coated is non-conductive, instead of theabove-described technique, a conductive anode, for example a metalliccarrier plate, is provided next to the body which is to be coated. Inall cases, between the object to be coated and the object to besputtered, lies the cathode potential drop space of the discharge, i.e.a path of at least '5 to 100 median, free path lengths, through whichthe sputtered material can be diffused. As a rule, a gas pressure of and10" torr is adjusted in the discharge, so that one can count on a medianfree path length of 10 to 10 cm. Due to these conditions, one obtainssmall sputtering rates and considerable impurities, in connection withmaterials which are difficult to atomize. Improvement can be achievedpartly by using hot cathodes and magnetic fields or special ion sourcesoutside of the highly evacuated space which contains the object to becoated. This type of method is not favorable due to its intricacies andits considerable expenditure.

The present invention deals with this problem.

The invention relates to a method of coating a layer of inorganic, solidmaterial upon a base, e.g. a semiconductor surface, by cathodesputtering. According to my invention, the method is characterized bythe fact that a grid or sieve-like perforated cathode, which iscomprised, at least at its surface, of the material to be deposited, isarranged between anode and substrate and is negatively biased againstboth. Furthermore, the substrate is brought so close to the cathode thatthe space between the two is insufficient to maintain an independent gasdischarge, at the adjusted pressure and the applied voltage.

-It is important that the perforated, e.g. grid-like, cathode isarranged at a distance of a maximum of approximately ten median freepath lengths, from the substrate to be coated, so that the cathode drop,which maintains the discharge, is located outside of the space betweensubstrate and cathode, namely between the latter and an anode located agreater distance away. Material, sputtered by the cathode, has,therefore, only a short diffusion path. Preferably, the grid rods of theperforated cathode Will 3 ,654,1 l0 Patented Apr. 4, 1972 have atape-shaped configuration, whose surfaces lie perpendicularly to thesubstrate surface. The shape of the cathode is similar, in this case, tothat of a grating of a shoe scraper. This improves the effectivenesseven more, the radiating cathode surface is enlarged and the uniformityof the atomized layer is not impaired.

In the drawing,

FIG. 1 schematically shows appropriate apparatus for performing themethod of a preferred embodiment of the invention;

FIG. 2 shows a preferred embodiment of the grid-type cathode, in anoblique top view; and

- FIG. 3 shows another embodiment of the invention.

The object 1 to be coated, e.g. a semiconductor crystal of silicon orgermanium, or the carrier for the conductor paths or resistor layers, isconnected to the positive pole of a direct current voltage source 2, atwhose negative pole is the cathode 3. An additional direct voltagesource 4 is between the cathode 3 and the anode 5. This arrangement,with the exception of the direct current voltage sources (e.g.batteries), is arranged in a vessel 6, for example made of glass, whichcan be evacuated. The distance d between the substrate 1 to be coatedand the perforated cathode 3 is less than ten median free path lengths,which at a preferable gas pressure p=2 10- torr inside the evacuatedvessel 6, is a path of 20 mm., and is preferably 10 mm. The distance Dbetween the cathode 3 and the anode 5 is 20 cm. During practicalapplication, the substrate 1 is located on a holder 7 which serves, atthe same time, as an electric lead and which makes it possible toregulate the distance d between substrate 1 and cathode 3. It is alsopossible to swing the cathode 3 out from the space between substrate 1and anode S to apply an appropriately high voltage between the anode 5and the substrate 1. In this manner the substrate is cleaned by exposureof its surface to ion bombardment. The process is preferably carried outat a continuous evacuation apparatus, just as it is sometimes expedientto effect the actual coating of the substrate 1 with a continuousevacuation pump. Connection to a vacuum pump (not shown) is indicated as8. Argon, for example, is introduced at 9 via a needle valve, whichregulates the gas pressure. To effect the actual coating, the cathode 3is then placed between substrate 1 and anode 5 and, in accordance withthe present invention, the dimensioning of the distance d is observed.The voltages between substrate 1, cathode 3 and anode 5 are so selectedthat in any event only one sputtering of the cathode surface takes placewhile such action is prevented at the surface of the substrate. At anappropriate dimensioning of the distance d a satisfactory coating quotais obtained at the surface of the substrate, even when the metalsinvolved are distinctly difficult to atomize, such as tungsten,molybdenum, chromium, tantalum, titanium, and aluminum. Semiconductorsand non-metals, as well as appropriately resistant chemical compoundssuch as oxides, nitrides, carbides, silicides and other such compoundscan be atomized or applied as coating. In cases where the conductivityof said materials is insufficient for maintaining an undisturbed cathodefunction, it is best to coat two electrodes with the respectivematerial, one of which is perforated in accordance with the inventionand is arranged immediately ahead of the substrate, whereby ahigh-frequency alternating voltage is connected between the two.

One must make sure, however, that all remaining parts in the recipientare always positively charged with respect to both electrodes. Anotherpossibility of atomizing or sputtering non-conductive chemicalcompounds, in the manner suggested by the present invention, is first toproduce said compounds in the gas chamber in such a way that aconductive component, e.g. silicon, is sputtered as a cathode in anon-inert gas, e.g. oxygen, nitrogen or their compounds, whereupon thesubstrate will be coated with the compound which results. Experience hasshown that an optimum voltage exists with a selection of d=1 cm., D: 20cm. and a cathode material comprised of nickel and argon, at 1-4 kv.

The perspective view of the cathode 3, illustrated in FIG. 2, showsplainly the preferred, tape or band-like structure of the individualrods which form the cathode. The tapes can also be oriented diagonallyto the surface of the substrate.

The coated layers obtained according to the method of the invention,particularly metal or semiconductor layers, have a particularly goodadherence if the distance is less than approximately one median freepath length. It is moreover recommended to move the cathode in parallelto the substrate surface, respectively to shift the substrate 1 back andforth beneath the cathode during the atomizing process in order toimprove, in this manner, the uniformity of the coated layer.

If the largest possible number of small parts, such as semiconductorcomponents produced with the planar method, glass or ceramic plates, areto be provided with a tightly adhering layer, then in accordance withFIG. 3, the carrier for these parts can be constructed as a sphericalsegment 7 whereby the space of the recipient will be utilized to theutmost. The grid-shaped perforated cathode 3 is then also curved. Inaccordance with the invention, the grid is arranged closely to theinside surface of the carrier segment 7. The metal wall 6 of therecipient which is at ground potential can serve as the anode and can bein conductive connection, if necessary, with the carrier segment 7. Thisarrangement also makes it possible for the cathode 3 to swing out of theway prior to or following the sputtering process and to place anevaporization source into the center point M of the carrier segment aswell as to vapor-depoist an additional material upon the parts 1 whichare to be coated.

I claim:

1. A method of applying a layer of inorganic solid material upon asubstrate, by means of a cathode sputtering, which comprises placing agrid-like perforated cathode, comprised at least at its surface, of thematerial to be sputtered, between the anode and the substrate, saidcathode being negatively biased with respect to both the anode andsubstrate, the substrate being so close to the cathode that notindependent gas discharge can occur between the two.

2. A method of supplying a layer of inorganic solid material upon asubstrate, by means of a cathode sputtering, which comprises placing agrid-like perforated cathode, comprised at least at its surface, of thematerial to be sputtered, between the anode and the substrate, saidcathode being negatively biased with respect to both the anode andsubstrate, the substrate being so close to the cathode that noindependent gas discharge can occur between the two, the distancebetween cathode and substrate being a maximum of ten median, free pathlengths of the particles, sputtered from the cathode.

3. The method of claim 2, wherein a perforated cathode whose band-likesurfaces are oriented vertically to the surface of the substrate to becoated, is used.

4. The method of claim 3, wherein at the onset of the process adischarge is effected between the substrate to be coated and the anodewith the cathode removed, whereby a cleansing occurs at the surface ofthe substrate, thereafter placing the cathode between the substrate tobe coated and the anode and applying the voltage needed for sputteringthe cathode material to coat the substrate.

5. The method of claim 4, wherein there is a relative movement, betweenthe cathode and the substrate to be coated, during the coating process.

6. The method of claim 5, wherein the voltage between anode and cathodeis l to 4 kv., and between the cathode and the substrate to be coated, 0to 10 kv.

7. The method of claim 6, wherein a gas pressure of l to 10- torr ismaintained in the cathode sputtering chamber and the gas discharge iseffected in the residue of argon and/or another inert gas.

8. The method of claim 7, wherein the geometrical configurations ofanode and cathode is such that a focussing takes place, of the positiveions emitted by the anode, upon the cathode.

9. The method of claim 8, wherein a metal semiconductor, or insulatingcoating, is further reinforced through vapor deposition.

10. The method of claim 8, wherein the discharge takes place in achemically active gas, which reacts with the cathode material, so that acoating of the substrate takes place with the resulting compound.

11. The method of claim 8, wherein a metal semiconductor or insulatingcoating is further reinforced by galvanic means.

References Cited Holland, Vacuum Deposition of Thin Flms, 1963, pp. 420,421, 426 and 427.

JOHN H. MACK, Primary Examiner S. S. KANTER, Assistant Examiner US. Cl.X.R. 204298

